1. Field of the Invention
The present invention relates to an integrated circuit having means for determining whether the channel lengths of field effect devices are outside a given limit.
2. Description of the Prior Art
Integrated circuits having field effect transistors are designed to obtain a desired channel length (distance between the source and drain) for each transistor. In most cases, the transistors having the shortest channel lengths have the highest transconductance (Gm). In digital integrated circuits, these devices are usually the fastest devices in terms of switching speed, and they often determine the overall speed of the integrated circuit. Similarly, in analog integrated circuits, the shortest channel length devices often determine the highest frequency at which the integrated circuit will operate. Therefore, it is desirable to be able to design and fabricate integrated circuits having at least some field effect transistors with relatively short channel lengths.
One limitation on channel lengths is the so-called "short channel" effect, wherein the channel becomes so short that the gate electrode can not exercise proper control of the current flowing between the source and drain. In that case, a larger change in the gate voltage is necessary to effect a given change in the channel current; i.e., the transconductance is reduced. Of even more importance in many cases is the reduction in reliability due to "hot carriers" that often accompanies the short channel effect. The exact channel length at which the short channel effect occurs depends in part on the various integrated circuit process operations, which determine the doping levels, junction depths, gate insulator thicknesses (in the case of insulated gate field effect transistors), etc. In addition, there are inevitable variations in the lithography technique used to define the channel lengths. Therefore, a given integrated circuit may have an undesirably short channel length, due to the various uncontrolled process variations that occur from the fabrication of one integrated circuit to another.
In order to test for short channel effects, it is known to include on an integrated circuit wafer test transistors having a channel length as short as that of the shortest functional transistors. During the wafer probe test operation, it is known to measure the drain-to-source current as a function of gate voltage, in order to determine short channel effects. It is also known to measure the gain (beta) of a series of test transistors having differing design channel lengths. An excessive gain indicates that the actual channel lengths are less than the design value. However, the test transistors are located between the integrated circuit chips, and extra time for probing the test transistors must be provided. As a result, only a few (typically five in one current process) of the test transistors are measured per wafer, and even that results in substantial additional test time.